1. Field of the Invention
The present invention relates to a single-ended output class-D amplifier, and more particularly, to a pop-free single-ended output class-D amplifier capable of controlling an output voltage when a power supply is turned on/off or a shutdown signal is activated/released for preventing the output voltage from rising and falling rapidly, in order to reduce a ‘popping’ sound.
2. Description of the Prior Art
As technology advances, consumer electronic products have become essential items in everyday life. This huge popularization of consumer electronic products has led to their quality requirements also becoming higher; hence, products are developed to be of high quality and refinement. An acoustic device is provided here as an example (e.g. an earphone or hi-fi equipment) . When listening to music or watching a movie, low quality sound or frequent ‘popping’ noises will significantly detract from a viewer's enjoyment. More and more portable electronic products are equipped with electric-to-audio converting devices. This can give rise to power saving and weight concerns, and therefore other issues such as low distortion, high efficiency, small size, and low cost must be considered when designing these devices.
Conventionally, an audio signal amplifier of the acoustic device can be implemented with an A-class amplifier, a B-class amplifier, or an AB-class amplifier. Amongst the three classes of amplifiers, the A-class amplifiers have the maximum static working currents and smallest distortion but have the lowest power efficiency, and thus generate a higher thermal energy. The B-class amplifiers have the minimum static working current and highest power efficiency but also the greatest distortion. The AB-class amplifiers combine the advantages of the A-class amplifiers and the B-class amplifiers. The AB-class amplifiers have static working currents which are between the A-class amplifiers and the B-class amplifiers, and the distortion and power efficiency also lie between both types of amplifiers. Therefore, most acoustic devices and electric-to-audio converting devices utilize the AB-class amplifiers as their audio signal amplifier.
In recent years, class-D amplifiers have become popular. Since the class-D amplifiers possess higher power efficiency in comparison with the A-class amplifier, the B-class amplifier, and the AB-class amplifier, heat radiators or other additional cooling equipment may not be required. This makes the class-D amplifiers suitable for high power application. Since portable products have become more popularized in recent years, the requirements for power saving and lightness have become more important, such that the D-class amplifiers, in which heat radiators are not required, have replaced the AB-class amplifiers as the mainstream for the audio signal amplifiers.
The output stage of the audio signal amplifier can be implemented with one of two types of structure: bridge-tied load (BTL) and single-ended. The implementation of the BTL type output stage is illustrated in FIG. 1A and FIG. 1B, which are schematic diagrams of an output stage 10 of an audio signal amplifier with different current directions in the conventional BTL structure. The output stage 10 includes high-side transistors 102 and 106, low-side transistors 104 and 108, inductors L1 and L2, capacitors C1 and C2, and an electric-to-audio converting device 115. The inductor L1 and the capacitor C1 construct a low-pass filter F1, and the inductor L2 and the capacitor C2 construct a low-pass filter F2. The high-side transistors 102, 106 are coupled to the inductors L1, L2 and a power supply terminal VCC, respectively, and the low-side transistors 104, 108 are coupled to the inductors L1, L2 and a ground terminal GND, respectively. As shown in FIG. 1A, a current I1 flows from the power supply terminal VCC via the high-side transistor 102, the low-pass filter F1, the electric-to-audio converting device 115, the low-pass filter F2, and the low-side transistor 108 to the ground terminal GND. At this point, if the left terminal of the electric-to-audio converting device 115 is a positive terminal and the right terminal is a negative terminal, the current I1 flows from the positive terminal to the negative terminal, such that voltage of the positive terminal is greater than voltage of the negative terminal. As shown in FIG. 1B, a current 12 flows from the power supply terminal VCC via the high-side transistor 106, the low-pass filter F2, the electric-to-audio converting device 115, the low-pass filter F1, and the low-side transistor 104 to the ground terminal GND. At this point, if the left terminal of the electric-to-audio converting device 115 is a positive terminal and the right terminal is a negative terminal, the current 12 flows from the negative terminal to the positive terminal, such that voltage of the negative terminal is greater than voltage of the positive terminal. With the currents switched between different directions, the electric-to-audio converting device 115 can generate sound.
The implementation of the single-ended output stage is illustrated in FIG. 2A and FIG. 2B, which are schematic diagrams of an output stage 20 of an audio signal amplifier with different current directions in the conventional single-ended output structure. The output stage 20 includes a high-side transistor 202, a low-side transistor 204, an inductor L1′, capacitors C1′ and C3, and an electric-to-audio converting device 215. The inductor L1′ and the capacitor C1′ construct a low-pass filter F1′. The high-side transistor 202 is coupled to the inductor L1′ and a power supply terminal VCC, and the low-side transistor 204 is coupled to the inductor L1′ and a ground terminal GND. As shown in FIG. 2A, a current I1′ flows from the power supply terminal VCC via the high-side transistor 202, the low-pass filter F1′, and the capacitor C3 to the electric-to-audio converting device 215. At this point, the current I1′ flows to a positive terminal of the electric-to-audio converting device 215, such that voltage of the positive terminal of the electric-to-audio converting device 215 may rise. As shown in FIG. 2B, a current I2′ flows from the ground terminal GND via the electric-to-audio converting device 215, the capacitor C3, the low-pass filter F1′, and the low-side transistor 204 back to the ground terminal GND. At this point, the current I2′ flows out of the positive terminal of the electric-to-audio converting device 215, such that voltage of the positive terminal of the electric-to-audio converting device 215 may fall. With the currents switched between different directions, the electric-to-audio converting device 215 can generate sound. As a result, in comparison with the output stage 10, which requires two sets of high-side transistors, low-side transistors, and low-pass filters, the output stage 20 requires only one set of high-side transistor, low-side transistor, and low-pass filter. The number of circuit elements can be saved, and circuit areas and pin numbers can also be saved, which reduces package size requirements and saves bonding wires to reduce package cost. In the output stage 10, the current drives the electric-to-audio converting device 115 via two sets of high-side transistors and low-side transistors. In comparison, in the output stage 20, the current drives the electric-to-audio converting device 215 with only one set of high-side transistor and low-side transistor. Power consumption in the transistors can be saved by half.
In comparison with the output stage 10, the structure of the output stage 20 may be more likely to generate the ‘pop’ sound. As shown in FIG. 1A and FIG. 1B, according to the structure of the output stage 10, both terminals of the electric-to-audio converting device 115 are coupled to similar circuit structures. When the power supply is turned on/off or other events occur to cause the output voltage to change rapidly, the voltage of both terminals of the electric-to-audio converting device 115 may change simultaneously, which will not generate a pop. In comparison, according to the structure of the output stage 20, when the output voltage changes rapidly, the voltage difference between both terminals of the electric-to-audio converting device 215 may change rapidly to generate a pop. To overcome this problem, the output stage 20 needs an additional control circuit. Fortunately, an area of the control circuit may be far less than an area of the high-side or low-side transistor in the output stage. The single-ended output structure, which needs only one set of high-side transistor, low-side transistor, and low-pass filter, is better than the BTL structure when cost is a consideration.
The conventional method for solving the pop issue is usually to utilize more resistors and capacitors in the reference voltage generator, or any method which increases the time constant to extend a bias setup time for the output voltage (e.g. extend to 1 second). When the bias of the output voltage is required to be set up in a shorter time (e.g. when the power supply is turned on/off or the shutdown signal is activated/released), the pop noise may occur, and the problem is especially severe under higher voltage. Therefore, there is a need to provide a method to reduce a pop when the bias of the output voltage is required to be set up in a shorter time (e.g. when the power supply is turned on/off or the shutdown signal is activated/released), for the single-ended output class-D amplifier.